Synthesis and investigation of dielectric ceramic nanoparticles for microstrip patch antenna applications

Zinc aluminate (ZnAl2O4) is a well-recognized ceramic demanded in several microwave applications. Further, the addition of dielectric materials in ZnAl2O4 improved its dielectric properties, which is promising for the realization of a microstrip patch antenna. This article reports the investigation of ZnAl2O4TiO2 (ZAT) dielectric ceramic nanoparticles synthesized by the sol–gel process. The X-ray diffraction analysis revealed the crystalline nature of the prepared nanoparticles, with a tetragonal structure of anatase-, and rutile-TiO2 phases coexisting with the cubic phase of ZnAl2O4. The estimated crystallite size of the dielectric ceramic is 13.3 nm. Transmission electron microscopy (TEM) micrographs demonstrated the spherical grains with their mean diameter of 14.75 nm, whereas the selected-area electron diffraction (SAED) pattern endorsed the crystallinity of the sample. Raman measurement revealed the vibrational modes in accordance with the TiO2 and ZnAl2O4 compounds. The dielectric properties of the ZAT sample showed the dielectric permittivity in the range of 22.12–21.63, with its minimum loss from 0.056 to 0.041. Finally, a prototype microstrip antenna was fabricated using the prepared nanoparticles, which demonstrated a return loss of − 30.72 dB at the resonant frequency of 4.85 GHz with its bandwidth of 830 MHz.

www.nature.com/scientificreports/ gains were − 30.65 dB and − 29.5 dB corresponding to the YZ and XZ planes with their resonant frequency of 5.84 GHz 10 . Because of its unique properties, zinc aluminate (ZnAl 2 O 4 ) spinel-type ceramic has become increasingly important in modern technology. This material is beneficial in various applications due to its synergetic properties, including excellent durability, moderate heat treatment, good thermal resistance, better mechanical robustness, and broad bandgap (3.8 eV) [11][12][13] . Additionally, this has been employed as a transparent conductor for ultraviolet radiation, detector, dielectric, and optical substances [14][15][16][17] . ZnAl 2 O 4 has also been demanded in the telecommunications industry for resonating, filtering, and oscillating wireless fax, mobile phones, GPS, military radar systems, smart transmission systems, and satellites. The dielectric properties of ZnAl 2 O 4 as microwave dielectric ceramic by adding TiO 2 are studied [18][19][20][21] . They reported the potential applications of ZnAl 2 O 4 TiO 2 in future microwave substrates and antennas. The addition of a small amount of TiO 2 to the ZnAl 2 O 4 resulted in an increased dielectric permittivity. Recently, the properties of ZnAl 2 O 4 ceramic based on either composite or doped with TiO 2 , Mg 2 TiO 4 − xSrTiO 3 , Co 2 TiO 4 , Mg 2 TiO 4, etc. are being investigated. The ZnAl 2 O 4 based ceramic is a well-known patch material for GPS or microwave substrates. Abdullah et al. fabricated and studied the performance of the patch antenna using the nanoparticles of (1 − x)ZnAl 2 O 4 − xSiO 2 22 . The crystallite size of this compound was estimated to be in the range of 39.79 to 44.34 nm, along with the dielectric permittivity of 8.57. This patch antenna showed its return loss of − 14.25 dB at the resonant frequency of 3.46 GHz with its bandwidth of 60 MHz. Kim et al. studied the various dielectric ceramics having a large dielectric permittivity (ε r ) and positive temperature coefficient of resonant frequency (τ f ) values 2 . It implies that low-dielectric ceramics (< 20) are critical for frequency stability across a range of temperatures. According to Narang and Shalini et al., it is possible to improve the properties of dielectric ceramics by incorporating an appropriate material and modifying the synthesis approach 23   This paper reports the synthesis and investigation of ZnAl 2 O 4 TiO 2 dielectric ceramic nanoparticles prepared by an in-expensive and straightforward sol-gel route. Further, we employed these nanoparticles to fabricate a prototype microstrip patch antenna, which demonstrated its return loss of − 30.72 dB at a resonant frequency of 4.85 GHz. To the best of our knowledge, no similar work of fabricating a microstrip patch antenna using ZnAl 2 O 4 TiO 2 has been reported for the C-band applications. "Materials and methods" section presents the materials and methods of synthesizing ZnAl 2 O 4 TiO 2 composite nanoparticles. The characteristics of ZnAl 2 O 4 TiO 2 nanoparticles, dielectric properties, and microstrip patch antenna performance are discussed in "Results and discussion" section. Lastly, "Conclusions" section summarizes the paper.

Materials and methods
For the sol-gel synthesis of ZnAl 2 O 4 TiO 2 (ZAT) dielectric ceramic nanoparticles, titanium tetra isopropoxide (TTIP, Sigma Aldrich), zinc acetate (CH 3 COO) 2 Zn 2 H 2 O (Lobychem), aluminum nitrate nonahydrate (Al 2 (NO 3 ) 3 ·9H 2 O, (Sigma Aldrich), ethanol (C 2 H 5 OH, Sigma Aldrich), ethylene glycol (EG, SdFine) and nitric acid (HNO 3 , Lobychem) were procured and used without any further purification. Figure 1 illustrates the step-by-step process followed in synthesizing the composite ZnAl 2 O 4 TiO 2 nanoparticles and the pellet preparation for the measurement of dielectric properties. Initially, 5 ml distilled water was added to 75 ml ethanol and stirred. Then 4.5 ml TTIP solution was added and stirred for about 4 h at approximately 85 °C. Followed by this, a white powder was obtained, calcined at 700 °C and ground. In a separate beaker, 15 g aluminium nitrate nonahydrate was added in 40 ml ethanol, and later 0.4 ml ethylene glycol (EG) was mixed in the above solution under constant stirring conditions. Finally, 6.6 g zinc acetate dehydrate and 0.32 g TiO 2 powder (previously prepared) were mixed in sequence to the solution mentioned above at temperature 75 °C and stirred for 1 h.
During this process, 0.24 ml nitric acid (HNO 3 ) was dropped into the solution for preparing the homogeneous solution and stirred at temperature 75 °C for another 1 h till the formation of a clear solution. Doing so, (1 − x)ZnAl 2 O 4 − xTiO 2 powder was obtained while x is the concentration of the TiO 2 (i.e. x = 0.1). The sample was dried in an oven for 30 min at a temperature of 180 °C. Lastly, the sample was calcined at a temperature of 700 °C for 1 h and then ground. The prepared nanoparticles were employed to prepare a pellet using the pellet press machine. The prepared pellet was 1.12 mm thick and 10 mm in diameter. Prior to dielectric measurement, the as-prepared pellet was thermally treated at temperature 700 °C for 1 h and then it's both sides were coated using the silver paste by doctor blade process. www.nature.com/scientificreports/ The prepared sample named as ZA was characterized by using an X-ray diffractometer (XRD, X-Pert Pro, UK), Raman spectroscopy (BWTEK, Japan), Transmission electron microscopy (TEM, TALOS F200S G2, USA), energy dispersive X-ray spectroscopy (EDS), and LCR meter (PSM1735 N4L, Newtons4th Ltd, UK). The fabricated prototype microstrip patch antenna based on composite nanoparticles was tested using the Vector Network Analyzer (VNA, R&S®ZVL, Germany).

Results and discussion
The X-ray diffraction (XRD) investigation is advantageous for determining the crystalline phases of the nanomaterials. Figure 2 depicts the XRD pattern of ZAT nanocomposite dielectric ceramic material. It represents the various peaks of ZnAl 2 O 4 crystal structure corresponding to the typical face-centered cubic morphology and was found consistent with the reported literature 30,31 . One can also notice the formation of the crystalline structure of titanium dioxide (TiO 2 ) with its anatase and rutile phases and the wurtzite structure of ZnO 32,33 . Our XRD result also coincides with the JCPDS File No. 00-021-1272 and 01-021-1276 of anatase and rutile TiO 2, respectively. Additionally, it matches well with the JCPDS File No. 00-005-0669 and 89-0510 corresponding to the ZnAl 2 O 4 and ZnO. Compared to the pristine ZnAl 2 O 4 sample, which showed the various peaks of ZnAl 2 O 4 , some peaks of ZnO can also be noticed 18,19 . Further, in the composite sample of ZnAl 2 O 4 TiO 2 the additional peaks of TiO 2 were noticed 34,35 . In addition, the peak locations were found slightly shifted with the increased value of TiO 2 concentration. In other words, the unit cell dimension was noticed to be decreased with the enhanced crystallinity 34,36 . In our case, the crystallite size was estimated to be 13.3 nm using the Scherrer formula (d = 0.94λ/(βcosϴ), where β is the X-ray wavelength, and β is the full-width at half-maximum intensity of the diffraction line). Figure 3 shows the Raman spectra of ZnAl 2 O 4 TiO 2 nanoparticles sintered at a temperature of 700 °C. We noticed various peaks from the prepared sample that originated due to ZnAl 2 O 4 , ZnO, and TiO 2 contents. We can observe two Raman peaks originated at 395 cm −1 and 519 cm −1 assigned to B1g and A1g/B1g modes, respectively, showing the impression of anatase-TiO 2 . Further, a peak 439 cm −1 known as E 2 high vibration mode was associated with oxygen atoms and assigned to ZnTiO 3 nanocrystals. A broad peak at 618 cm −1 represents the thermodynamically stable rutile-TiO 2 relates the space group D 4h assuming the site symmetries for the Ti and O atoms within the unit cell. In literature, this peak is attributed to the Raman-active "lattice vibration" designated as A 1g 37-39 . TEM measurement was carried out to know the morphology of ZAT dielectric ceramic nanoparticles.   Figure 5a illustrates the histogram of ZnAl 2 O 4 TiO 2 nanoparticles by measuring the diameters of about 40 grains to estimate the average size. As can be seen, the dielectric ceramic nanoparticles have an average diameter of 14.75 nm.
The EDS spectrum of ZnAl 2 O 4 TiO 2 dielectric ceramic nanoparticles is shown in Fig. 5b. The EDS spectrum revealed the elemental peaks of O, Zn, Al, and Ti at energy values 0.52, 1.11, 1.48, and 4.5 keV, respectively.
The dielectric permittivity value represents the material's ability to store electric energy when an electric field is applied, and it is related to the capacitance associated with the dipole orientation of charge carriers. After obtaining the parallel capacitance values, we have calculated the dielectric permittivity by using an expression, ε r = Cd/ε o A , where C is the capacitor's capacitance, d is the pellet's thickness, ε 0 is the permittivity of free space, and A is the cross-section area of the pellet. The dielectric characteristic of ZnAl 2 O 4 TiO 2 nanoparticles was studied using the LCR meter with its frequency range from 100 Hz to 1 MHz at room temperature. Figure 6 shows the dielectric measurement setup used in this study. The LCR meter is connected to the computer, while the front panel (bottom-left) of the LCR meter is loaded with a pellet under the test. One can also do the dielectric measurement by varying the temperature through a separate unit, as shown here. However, the dielectric measurement was carried out at room temperature in this case. Figure 7 depicts the variation of dielectric permittivity in accordance with the frequency. With the increased frequency, one can observe the decreased permittivity. The dielectric permittivity was varied from 22.12 to 21.63  www.nature.com/scientificreports/   20 . An abrupt decrease in dielectric permittivity in the lower frequency range was noticed, which was found constant in the higher frequency region. This typical characteristic of such materials can be attributed to the reduced polarization 39 . Dielectric loss (tanδ) is an important parameter representing the energy dissipation, and therefore, it needs to be studied. Dielectric loss is also regarded the microstructure faults, e.g. microstructural defects, porosity, microcrashes, the spontaneous orientation of crystallite, etc. 21 . Figure 8 depicts the dielectric loss of ZnAl 2 O 4 TiO 2 nanocomposite ceramic sample as a function of frequency. The dielectric loss was noticed to be decreased from 0.056 to 0.041 with an increased frequency range from 100 kHz to 1 MHz. In general, one can observe the reduced dielectric loss with the increased frequency. This nature is because the hopping ions lag behind the applied electric field. At the lower frequency range, one can notice the increased dielectric loss value, which further decreases in the higher frequency region. In the first case, this relates to the high resistivity resulting from the associated effect of grain boundaries.
We have calculated the real (Z′) and imaginary (Z″) impedance values with respect to the frequency, which are plotted in Figs. 9 and 10, respectively. As shown in Fig. 9, we can notice the decreased real-impedance from 3.28 kΩ to 467 Ω with the rise in frequency. Similarly, Fig. 10 depicts the same trend of increased imaginary impedance values from − 59 to − 10 kΩ with the increased frequency from 100 kHz to 1 MHz. The Z′ value fluctuates with temperature and joins together in the higher frequency region (not shown here). This happens due to the    www.nature.com/scientificreports/ liberation of charge carriers and semiconducting characteristics at high temperatures 40 . However, the Z′′ value rises with temperature, and the larger value at a higher frequency regime indicates the increase in tangent loss. Figure 11 illustrates the frequency-dependent ac conductivity at room temperature. The ac conductivity was estimated using the relation σ ac = ωεε o tanδ, where ε o is the free space dielectric permittivity, ε is dielectric permittivity, ω is the angular frequency, and tanδ is the tangent loss. The investigation of conductivity as a function of frequency relates to the process of charge transport. We can notice the enhancement in conductivity from 2.2 × 10 -5 to 9.8 × 10 -5 with the increased frequency. This increasing trend of conductivity in the lower frequency range (not shown here) can be attributed to space charges scattering cations across adjacent sites 41 . The conductivity curve coincides at high-frequency band, representing that the conductivity curves obey Jonscher's power law and therefore exhibit low-frequency dispersion phenomena 42 .
The prepared ceramic dielectric ZnAl 2 O 4 TiO 2 nanoparticles were employed for preparing a patch antenna. Initially, ZAT paste was prepared, which was cast on the FTO substrate and then it was silver coated on both sides for metal contacts. Finally, the SMA connector was connected to it, and antenna performance was evaluated using a vector network analyzer. Figure 12 depicts the patch antenna's top view which illustrates its dimension and shape. The fabricated prototype microstrip patch antenna has its length and width of 25 mm and 15 mm, respectively, as illustrated in Fig. 12a.

Conclusions
Dielectric ceramic ZnAl 2 O 4 TiO 2 nanoparticles prepared using the low-cost and easy technique have been studied. The synthesized nanoparticles were crystalline with their crystallite size of 13.3 nm. The Raman study evidenced the corresponding Raman shift of the constituent elements presented in the composite nanoparticles. The morphological investigation of the nanoparticles endorsed the formation of spherical grains with their mean diameter of 14.75 nm. The crystallinity of the prepared sample studied by the SAED pattern was consistent with the XRD result. The LCR meter measurement showed the decreased dielectric permittivity and loss as a function of